The resolution to this issue is often making sure you have entered a credit card or other valid method of payment* even if you don't plan on needing to use it. This is very likely in this case since you don't have enough balance to pay for the item.
If there is an option to select "Review" or "Continue", select that and enter valid payment information if required. If there's problems doing this:
On a mobile device tap: Settings > [your name at the top] > Media & Purchases > View Account > Review Apple ID.
Alternatively this document explains how to 'Change, add, [review,] or remove Apple ID payment methods' on an Apple mobile device, a Mac, or a PC; click here --> https://support.apple.com/HT201266
More things to try in this 2018 post by Urquhart1244 marked Apple Recommended:
https://discussions.apple.com/thread/8434425?answerId=33542414022#33542414022
If you continue to have problems contact Official Apple Support - Select your region from the list and then your country from the pop-up window. If you do not see your country, see if there is an "other" listing for that region.
https://getsupport.apple.com/?caller=cups&PGF=PGF63005
*Payment methods that you can use with your Apple ID - https://support.apple.com/HT202631
A change in amount could be die to tax being added.
